WO2014/096265 described an autonomous underwater vehicle (AUV) for recording seismic signals during a marine seismic survey. The AUV includes a body having a flush shape; a buoyancy system located inside the body and configured to control a buoyancy of the AUV while traveling underwater; a seismic sensor for recording seismic signals; and a processor connected to the buoyancy system and configured to select one of plural phases for the buoyancy system at different times of the seismic survey.
The buoyancy system comprises a chamber, and in a first phase water is introduced into the chamber and air released from the chamber so that the buoyancy of the AUV becomes neutral or slightly negative and the AUV starts its journey towards a target position at the sea bottom. When the AUV reaches the target position, the AUV needs to make a good coupling with the bottom so that the seismic sensor(s) record a high quality seismic signal. To achieve this goal, and also to stabilize the AUV on the sea bottom, more water is introduced into the chamber and air released. In this second phase, the buoyancy of the AUV is made negative, thus increasing the coupling with the sea bottom. In a third phase, compressed gas is introduced into the chamber to displace the water and make the AUV positively buoyant so it detaches from the seabed and floats towards the surface.
A problem with the AUV of WO2014/096265 is that the active buoyancy system and associated processor are complex and expensive. Also, if the AUV is to be operated at extreme depth—for instance at 3000 m—then the compressed gas must be at a very high pressure—for instance 30 MPa. Such a highly pressurised gas presents a safety hazard.